As described in copending US patent application No. 08/745,168 entitled "Configurable chirp Mach-Zehnder Modulator` to the present inventor (which is incorporated herein by reference) provision of higher transmission capacity in communication networks at competitive costs requires optimum utilisation of optical fiber transmission links. Optical transmission may be limited by effects such as chromatic dispersion, and spectral broadening, due in part to a wavelength shift, or chirp, caused by modulation of a laser source. A proper amount of chirp of an optical signal is important to achieve long haul and high bit rate transmission.
An alternative to directly modulated lasers is to use an electro-optical modulator which, in conjunction with a C.W. operated laser, offers the promise of a modulated signal with controllable chirp.
Prior art Mach-Zehnder modulators are commonly fabricated in lithium niobate (LiNbO.sub.3) with titanium (Ti) diffused waveguides. More recently Mach-Zehnder modulators have been fabricated in III-V material such as InP with multi-quantum-wells of InP/InGaAsP in the guided regions. Such a device is described by Rolland et al ("10 Gb/s 120 km Normal Fiber Transmission Experiment Using a 1.56 .mu.m Multi-Quantum-Well InP/InGaAsP Mach-Zehnder Modulator", Conf. Optical Fiber Communication, San Jose, Calif. 1993) in which some frequency chirp regulation is possible.
A Mach-Zehnder phase modulator is based on an interferometric configuration which converts phase modulation into intensity modulation. When the differential phase shift between the two arms of the modulator equals .+-..pi., the anti-symmetric mode is excited near the output and is subsequently diffracted out of the single mode waveguide. This is the `off` or logic `0` state. With no differential phase shift, the fundamental mode is excited and propagates with little loss to the output. This is the `on` or logic `1` state. The ability to control the frequency chirp of Mach-Zehnder modulators by varying the drive voltage to the arms or by a power splitting ratio has made them well suited for multi-gigabit long distance optical fiber transmission.
Mach-Zehnder III-V multi-quantum-well (MQW) modulators are increasingly attractive relative to other modulator designs because of their adjustable frequency chirp, low drive voltage, small size, long term reliability and potential for integration with laser sources. In III-V MQW Mach-Zehnder modulators where the two arms of the interferometer are of equal length with a power splitting ratio of 50%, the non-linear electro-optic effect imparts a small positive chirp under symmetric push-pull operation. While negative chirp can be induced by designing the y-junction to inject less optical power into the more deeply biased modulator arm and employing a small amount of overdrive, the back-to-back extinction ratio is degraded. This is described in greater detail in U.S. Pat. No. 5,524,076, issued Apr. 6, 1996, to Rolland et al.
The drive voltage which controls the differential phase shift is conventionally supplied to an electrode on one arm (single arm drive) or to both arms (dual arm drive) in a push-pull mode. Single arm drive requires a larger operating voltage than dual arm drive and generally provides more chirp than operation in a push-pull configuration. For a LiNbO.sub.3 modulator, the dual arm configuration driven with equal push-pull voltage gives substantially zero chirp.
A LiNbO.sub.3 Mach-Zehnder differs from a III-V MQW device because the latter shows a non-linear phase change as a function of bias voltage. In a LiNbO.sub.3 Mach-Zehnder absorption increases with voltage applied to the arms while absorption is not present in a III-V semiconductor Mach-Zehnder modulator. The amount of absorption in the III-V device depends on how close the operating wavelength is to the exciton peak of the multi-quantum-well material.
Because the Mach-Zehnder modulator converts phase modulation into intensity modulation it is important that the ratio between the `on` state and `off` state is relatively high. This ratio, also known as the extinction ratio (ER), is a measure of the signal intensity against background noise. Consequently, a high extinction ratio also permits a greater span between repeaters in a transmission network.
Positive frequency chirp adversely affects long haul transmission over non-dispersion shifted fiber. In such situations, a controlled level of negative chirp provided by the optical modulator is advantageous. Some transmission networks make use of dispersion shifted or dispersion compensating optical fibers in which case negative chirp is not required, and in fact it may be more appropriate to provide the data pulses with a slight positive chirp. There is, therefore, a requirement for optical modulators capable of selectively providing a desired amount of positive or negative chirp.
In U.S. Pat. No. 5,694,504 issued Dec. 2, 1997 to the present inventor, entitled "Semiconductor modulator with .pi. shift" which is incorporated herein by reference, a multi-quantum-well Mach-Zehnder optical modulator having different optical lengths between respective paths is described. The optical paths have a length differential equal to a phase difference of .pi., or an integral odd multiple thereof, at the selected operating free space wavelength. This differential in optical path length means that the interferometer is in the "off" or logic "0" state in the absence of electric field and "on" or logic "1", with electric field applied. This results in negative chirp with a good extinction ratio.
Copending US patent application Ser. No. 08/745,168, to the present inventor, referenced above, describes a .pi. phase shifted Mach-Zehnder modulator with one additional control electrode to provide further adjustment of phase by application of an electric field, whereby a configurable chirp, either positive or negative, may be obtained in a modulator having a built in .pi. phase shift. This device is an InP/InGaAsP MQW Mach-Zehnder modulator with a structure substantially identical to that discussed in U.S. Pat. No. 5,694,504, except that the optical path through one modulator arm is elongated to produce the required relative .pi. phase shift at the .lambda.=1.56 .mu.m operating wavelength and an additional control electrode is provided. Operation of a Mach-Zehnder modulator having .pi./2 phase shift and an additional pair of control electrode to provide increased flexibility of continuously tuning chirp is also described. A device having .pi./2 phase shift and two control electrodes provides either good positive chirp and a good extinction ratio or alternatively good negative chirp and a good extinction ratio in the same device.
On the other hand, the range of chirp, and particularly the maximum amount of the negative chirp obtainable, may not be sufficient for some communications applications. Operation of these devices with pairs of modulation electrodes is typically based on symmetric push-pull drive to avoid complicated control electronics. Operation of these devices with unequal push-pull voltages on the modulation electrodes requires an asymmetric drive to provide different voltages to each modulation electrode, which further complicates the control electronics. Consequently, it is desirable to provide further improvements to optmize or simplify control of chirp in Mach-Zehnder modulators.